Infrared imagers are becoming increasingly important for commercial and non-commercial applications such as night vision imaging. However, the imagers are still not widely used due to cost, size, weight and power consumption. For example, traditional night vision imagers, such as cryogenically cooled HgCdTe or InSb focal plane array imagers, uncooled bolometer or pyroelectric thermal imagers, require intervening electronics to convert the absorbed infrared radiation to a visible display. These additional electronics and display apparatuses incur additional cost, along with increased size, weight and power consumption.
There are a few proposed solutions to remove and/or minimize the electronics to the display for infrared imagers. For example, a direct view infrared structure was disclosed in U.S. Pat. No. 6,140,646, by Heinz Busta et al wherein an electrode on a cantilever, made of two layers of material or bi-material, with different thermal expansion coefficients, is used to modulate electron emissions from an array of field emissive devices. These emitted electrons then bombard a phosphor plate, giving a visible image. When IR radiation is incident on the bi-material cantilever, which usually consists of a layer of conductive film and a layer of insulator having different thermal expansion coefficients, the cantilever bends as its temperature rises in response to absorbed infrared radiation. This bending changes the distance between an electrode (the conductive film) and an associated emitter. As a result, the electric field established between that electrode and associated emitter changes and the amount of emitted electrons is modulated. There are a few obvious disadvantages to this approach. Among these, field emitters generally requires a large voltage, (a turn-on voltage), to achieve significant emission. The turn-on voltage is on the order of a few tenths of volts with extremely sharp emitters and small emitter-gate distance that are less than 1 μm. This turn-on voltage is not standard to CMOS technology. The turn-on voltage also creates an electrostatic force between the cantilever and the substrate. This electrostatic force may pull or collapse the cantilever if the turn-on voltage cannot be kept low. Second, field emission devices usually are not operated in a DC mode but in duty cycles or pulse mode to preserve the lifetime of the emitters and to establish emission stability. This requirement creates extra difficulties for the electronics. Third, moving parts always add uncertain reliability to the system, especially in a harsh environment. Fourth, the radiation has to pass through the substrate in order to reach the cantilever structure and, as a result, at least 50% of the radiation is lost. Fifth, high vacuum packaging is needed for field emission devices to provide a mean-free-path for the emitted electrons to reach the phosphor plate.
Another proposal was disclosed in U.S. Pat. No. 6,028,323 by Hui-Chun Liu, wherein the intensity of a LED is modulated by a quantum well infrared photodetector (QWIP) to provide a direct conversion from infrared to near infrared or visible radiation. Both a LED and an QWIP are fabricated on a GaAs-based semiconductor substrate. When infrared radiation illuminates the substrate, a fraction of the radiation is absorbed by the QWIP. The electrical resistance of the QWIP varies as a result according to the amount of absorbed radiation. This variation modulates the intensity of the LED. A CCD camera is used to capture emitted radiation from the LED. This proposal suffers a few drawbacks. First, the operation of QWIPs requires cryocoolers, which are sizable and power hungry. Second, the LED of this proposal is buried under the surface. Emitted light from the LED is severely diverged when it reaches the top layer. This makes the coupling between the QWIP-LED to the CCD camera difficult. Third, at least 50% of the incident radiation is lost in the substrate before reaching the QWIP. A third proposal was addressed in U.S. Pat. No. 6,080,988, wherein an optically reflective surface on a cantilever, which has a bi-material configuration with different thermal expansion coefficients for each layer, is used to modulate a reflected light beam. When IR radiation passes through the substrate and is absorbed by the bi-material cantilever, the bi-material cantilever bends as its temperature rises in response to absorbed IR radiation. A visible light source illuminates the reflective surface with a beam. The bending of the cantilever changes the angle of reflection of beam. One reflected beam has a different intensity than a beam reflected at a different angle when observed at a distant point. This method has no electronics and it requires no wiring. However, similar to the previous approaches, this method carries the shortcomings of power loss in the substrate and unreliability of moving parts. Due to extremely small bending of the cantilever, either the size of the overall setup is large or multiple lens and pinholes are required in order to obtain a viewable image.
Accordingly, an improved night vision imager which provides a direct visible image when converting infrared radiation, which has no moving parts and has an infrared absorber directly facing the incident radiation, and a LED array directly emitting light from its surface without any optical diverging effects, requiring no cryocooler, could have a major impact on future imaging and surveillance systems.